6SigmaET Tips: Auxiliary Variables; What Are They and When Do You Need Them?

4 April, 2017

Anybody who has ventured into 'Solution Control' may have come across a section called ‘Auxiliary Variables’. The name doesn’t give much away and you could be forgiven for wondering ‘what are they and when do I need them?’

What are 'Auxiliary Variables'? Well, they are variables that aren’t needed in most cases so are not stored by default to save memory. They are total pressure, density and heat fluxes. You can choose to store them by ticking the relevant box under Solution Control > Auxiliary Variables.

First on the list is total pressure. Total pressure is the sum of static and dynamic pressure. Plotting total pressure can be useful when trying to understand resistance to flow through a system; areas where resistance is high will exhibit a large drop in total pressure. In a fan driven system, minimising the loss of total pressure minimises the amount energy the fan needs to add to the flow. 

Next up is density. One scenario where you may wish to plot density is when 'Allow Ideal Gases' is turned on; you can then observe variation in density on a result plane.  Another scenario is when considering liquid cooling. When running a multifluid model it is good practice to check whether the closed liquid cooling loop leaks. This can be done by storing density, running a single iteration and then plotting density on a result plane. The image below shows a case with no leak; you can clearly see the difference in density between the water and the air. If you’re finding water where you expect to find air, for example, then you’ve got a leak. Unfortunately, plotting density can’t show you where a leak is coming from. However, here is some general modelling advice: Leaks most commonly occur where one object meets another - for example, where a cooling duct meets a solid obstruction. Making sure objects overlap (instead of just touching) often stops leaks occurring. 

Density cropped.PNG

The final auxiliary variable is heat fluxes. Heat flux, or more specifically heat flux density, is the amount of energy that flows through a unit area per unit time. This can be plotted on the surface of an object or on a results plane as a flow pattern or variation plot. Surface heat fluxes can be useful to understand how heat is dissipated from a component to the air. The image below shows the surface heat fluxes for a heat sink; heat fluxes are highest at the front where the temperature differential between the heat sink and the air is greatest.  Heat flux flow pattern can be used to identify bottlenecks where heat transfer is not effective. 

Pin Fin Heatsink.PNG